Kuratani and Ahlberg Zoological Letters (2018) 4:1 DOI 10.1186/s40851-017-0083-6 REVIEW Open Access Evolution of the vertebrate neurocranium: problems of the premandibular domain and the origin of the trabecula Shigeru Kuratani1* and Per. E. Ahlberg2 Abstract The subdivision of the gnathostome neurocranium into an anterior neural crest-derived moiety and a posterior mesodermal moiety has attracted the interest of researchers for nearly two centuries. We present a synthetic scenario for the evolution of this structure, uniting developmental data from living cyclostomes and gnathostomes with morphological data from fossil stem gnathostomes in a common phylogenetic framework. Ancestrally, vertebrates had an anteroposteriorly short forebrain, and the neurocranium was essentially mesodermal; skeletal structures derived from premandibular ectomesenchyme were mostly anterior to the brain and formed part of the visceral arch skeleton. The evolution of a one-piece neurocranial ‘head shield’ in jawless stem gnathostomes, such as galeaspids and osteostracans, caused this mesenchyme to become incorporated into the neurocranium, but its position relative to the brain and nasohypophyseal duct remained unchanged. Basically similar distribution of the premandibular ectomesenchyme is inferred, even in placoderms, the earliest jawed vertebrates, in which the separation of hypophyseal and nasal placodes obliterated the nasohypophyseal duct, leading to redeployment of this ectomesenchyme between the separate placodes and permitting differentiation of the crown gnathostome trabecula that floored the forebrain. Initially this region was very short, and the bulk of the premandibular cranial part projected anteroventral to the nasal capsule, as in jawless stem gnathostomes. Due to the lengthening of the forebrain, the anteriorly projecting ‘upper lip’ was lost, resulting in the modern gnathostome neurocranium with a long forebrain cavity floored by the trabeculae. Keywords: Head mesoderm, Cranium, Cyclostomes, Neural crest, Evolution, Trabecula Introduction homologues of vertebrae [2–4]. Gegenbaur and Froriep Ever since Rathke [1], who found a discontinuity be- thought that only the posterior part of the gnathostome tween the rostral and caudal part of the cranium, a part neurocranium, which lies alongside the notochord, could of the gnathostome neurocranium known as the trabec- be compared with the vertebral column; the neurocra- ula cranii has drawn the attention of vertebrate mor- nium could thus be divided into prespinal and spinal phologists (Fig. 1). The difference between the anterior parts [5, 6]. and posterior portions of the neurocranium resides pri- As recognized by Rathke, the above noted two regions marily in their morphological relationships; specifically, initially arise as two pairs of rod-like cartilages, trabecu- whether or not the cranial base is medially associated lae and parachordals (Fig. 1). It was Thomas Huxley who with the notochord, in a manner similar to the vertebral first put forth the idea that the trabecula might represent column. The morphological interpretation of this div- a highly modified element originally belonging to a ision is profoundly related to the segmental theory of visceral arch situated rostral to the mandibular arch the vertebrate head, developed from Goethe’s and Oken’s (premandibular arch) [7]. In the early twentieth century, ideas about the skull being composed of serial the original idea of Huxley was elaborated by many mor- phologists, including Goodrich and Sewertzoff [8–10]. * Correspondence: [email protected] De Beer also illustrated a schematic diagram to explain 1Laboratory for Evolutionary Morphology, RIKEN, 2-2-3 Minatojima-minami how an ancestral amphioxus-like animal evolved into Chuo-ku, Kobe, Kobe 650-0047, Japan the gnathostome morphotype (Fig. 2a) [11]. This Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Kuratani and Ahlberg Zoological Letters (2018) 4:1 Page 2 of 10 Fig. 1 a and b Schematized composition of crown gnathostome cranium. Early (a) and late (b) stages of development. Precartilaginous condensation and cartilages are shown in dots. Based on reference [8]. c and d. Early (c) and late (b) stages of chondrocranium in Salmo. Based on reference [42]. e, eye; eth, ethmoidal plate; Fen. bas.-caps., basicapsular fenestra; Fe, hypophys., hypophyseal fenestra; nas, nasal capsule; ns, nasal sac; nt, notochord; occ, occipital cartilage; opt, optic capsule; otc, otic capsule; ov, otic vesicle; pc, parachordals; tr, trabecula; ve, vertebrae ancestor is assumed to have possessed a series of visceral vertebral column (Fig. 3). Thus the neural crest/mesoderm arches along the anteroposterior axis, and these arches distinction does not coincide with the functional distinction showed no clear differentiation. Of these, the rostral of the neurocranium versus viscerocranium. It was further three arches represent prospective premandibular, man- suggested that the paraxial mesoderm requires a signal em- dibular and hyoid arches. On the other hand, the shark anating from the notochord to differentiate into the skeletal pharyngula-like gnathostome embryo has the trabecula, tissues, whereas the cephalic crest cells can differentiate functioning as a floor for the expanded forebrain, de- without this signal. On this basis, Couly and his colleagues rived from the premandibular arch. The ammocoete designated the anterior and posterior parts of the neurocra- larva-like creature is placed in the middle as an inter- nium as the prechordal and chordal cranium, respectively mediate stage, where the premandibular arch is posi- [12]. The two types of mesenchymal cell lineages corres- tioned rostral to the mandibular arch. pond perfectly to the presence/absence of the notochord, Toward the end of the 20th century, the concepts devel- distinct molecular basis of developmental signaling, as well oped by Gegenbaur and Froriep gained support from ex- as two types of the neurocranial anlagen. Moreover, the perimental embryology, which showed that the trabecular neural crest generally contributes to the visceral part of the part of the neurocranium is of cephalic neural crest origin cranium (visceral arch skeletons), making the premandibu- [12, 13], and only the notochord-associated posterior part lar arch-hypothesis of Huxley [7] appear somewhat plaus- of the neurocranium is of mesodermal origin, like the ible, although the basic idea of ‘arches rostral to the Kuratani and Ahlberg Zoological Letters (2018) 4:1 Page 3 of 10 Fig. 2 Evolution of the vertebrate neurocranium and origin of trabecula. a Scenario by de Beer. From top to bottom, amphioxus-like vertebrate ancestor, ammocoete larval-like intermediate state, and gnathostome morphotype resembling an elasmobranch pharyngula. The trabecula is assumed to represent the premandibular arch, which was secondarily incorporated into the neurocranium of the gnathostome whose forebrain is expanded enormously. Based on reference [11]. b The neurocranium of the adult lamprey, which is primarily formed of an inverted U-shaped ‘lamprey trabecula’, rostrally connected with its counterpart by means of the rostralmost portion called, in the present paper, the ‘transverse commissure’. The latter part is assumed to be of premandibular mesoderm and homologous with the orbital (acrochordal) cartilage of jawed vertebrates, and the rest of lamprey trabecula is thought to correspond to a rostrally extended parachordal. This figure is based on reference [31]. Abbreviations: hy, hyoid arch; II, optic nerve; IX, glossopharyngeal nerve; lamptr, lamprey trabecula; ma, mandibular arch; mo, mouth; nas, nasal capsule; nc, notochord; otc, otic capsule; pm, hypothetical premandibular archbyDeBeer[11];trc,transversecommissure;V1,ophthalmicnerve; V2 + 3, maxillomandibular nerve; VII, facial nerve; X, vagus nerve mandibular arch’ is currently not supported by many mor- preparation, and more recently synchrotron microtomo- phologists, mainly due to the apparent lack of pharyngeal graphy [17–22]. pouch-like structures rostral to the mandibular arch. Thesefossilsprovidedetaileddataontheshapeofthe Although this theoretical framework for the interpret- cranial cavity, including the position and approximate ation of the gnathostome neurocranium has a prominent morphology of the nasal sacs, inner ears, hypophysis, pineal evolutionary dimension, made explicit in De Beer’s sche- and parapineal organs, cranial nerves and cranial blood ves- matic diagram (Fig. 2a), it was developed almost entirely sels, as well as external features of the neurocranium, such without reference to the fossil record. During the late as the articulation points for the visceral arch skeleton. To- 19th and early twentieth century, fossils of primitive gether these structures create a network of landmarks that armoured jawed vertebrates (placoderms) and jawless can be used for drawing inferences about neurocranial